The metabolism of cholesterol, an essential cell membrane component and precursor in metabolic pathways, is tightly regulated at both the cellular and organismal level. Insufficient or excessive cholesterol levels result in pathologic processes, thereby necessitating a complex homeostatic regulation of cholesterol availability to cells and tissues. However, it remains poorly understood how the intricate processes of cholesterol influx and efflux are coordinately regulated to maintain cholesterol homeostasis. MicroRNAs are small endogenous RNAs that have emerged as post- transcriptional regulators of physiological processes. These short, double stranded RNAs are encoded in the genome and bind to complementary target sites in the 3' untranslated regions (3'UTR) of mRNAs, causing translational repression and/or mRNA destabilization. Notably, a single miRNA can have multiple targets, thus providing a mechanism for simultaneously regulating the post-transcriptional expression of genes involved in a specific pathway or physiological process. We hypothesize that microRNAs may play important roles in the epigenetic regulation of cholesterol metabolism gene pathways. To gain insight into role of microRNAs in cholesterol metabolism, we undertook an unbiased genome-wide screen of microRNAs modulated by cellular cholesterol content. We identified miRNAs that target components of the pathways regulating both low density lipoprotein (LDL) and high density lipoprotein (HDL). In this grant, we propose to determine the role of these microRNAs in both the physiological and pathological processes of cholesterol regulation. Specifically we will use overexpression and antagonism approaches to: (1) assess the role of these microRNAs in regulating lipoprotein metabolism, and (2) determine their impact on atherosclerosis progression and regression. These studies will provide insight into the mechanisms by which microRNAs contribute to the post-transcriptional regulation of cholesterol homeostasis, and assess their potential as therapeutic targets in the treatment of cardiovascular disease. !